About to take on more responsibility at work, best ways to approach a raise...

Document what the increase in responsibilities and then go out and find the pay ranges for that position, and your current position. Numbers are a great influencers.

I hope the best for you but I am guessing you are going to get screwed on this, since you were likely underpaid already. Why would they give you much of a raise.

if the numbers don't work, ask him what you need to do to prove your worth. Document that and revisit in 6 months.

Was in a similar situation recently. I work for a very large company and the person above me was leaving for a different position. They expected me to take on the load (which I was already taking on in expectation of getting the position when he left), while keeping my current position. All the while helping to train his replacement.

I walked into my manager's office and made my demands. My direct manager is a really good guy who completely understood everything I was saying. But at the time, I still didn't get the raise. My company basically gives you raises when you've reached new experience levels and that's it. I'm now casually looking around to see what other oppurtunities are out there. I think they've picked up on the fact that I'm not going to just take no for an answer, because now (3 months later) I'm getting an additional 2% on my yearly raise. Asking doesn't always lead to immediate results, but it's at least helped out a small amount over time. Good luck